The invention relates to improved cationic latexes useful in coatings and to a process for imparting mechanical stability to such latexes.
Although water-borne cathodic resin systems are well known, the use of latex binders in cathodic electrocoating is quite new and has not yet achieved full commercial acceptance. Copending application Ser. No. 513,621 filed July 14, 1983, now U.S. Pat. No. 4,512,860, gives background information on electrocoating latex for cathodic deposition and is incorporated herein by reference. The term "latex" is defined as a polymer or copolymer prepared from one or more monomers in an aqueous environment by emulsion polymerization techniques. Such latex, usually having an average particle size from about 800 .ANG. to about 10,000 .ANG. and an average molecular weight ranging from about 10,000 up to and above 1,000,000, is quite different from the water-reducible or ionizable polymers prepared, not in aqueous medium, but in solvent systems or neat. The latex water-reducible, cation-active polymers have been in commercial use for electrodeposition coatings for some time. Such conventional cationic latexes are known to be inherently less stable than their anionic counterpart systems.
For electrodeposition coatings, it is necessary to develop a latex that can be pumped and will be relatively insensitive to shear forces. Stability to shear is a necessary property for the latex itself, pigmented electrocoating compositions and for replenishment baths. In a commercial operation, the coating is continuously sheared by centrifugal pumping which passes the material through ultrafiltration membranes at a rate of 35-40 gallons/min. Instability of the coating, leading to agglomeration of particles after shear, causes fouling of the ultrafiltration membrane, application problems, and loss of coating properties. Such latexes tend to destabilize when subjected to shear forces and pumping processes. It is believed that destabilization is the result of a first-stage agglomeration of latex particles which, if uncontrolled, results in severe gellation. Gellation interferes with pumping and purification operations and usually interrupts the electrocoating process.
The closest art appears to be the coassigned patent application Ser. No. 716,665, filed Mar. 27, 1985. U.S. Pat. No. 3,640,935 (Abriss) teaches a method of improving latex stability by adding to the latex a non-ionic surface-agent (0.5-3%) as a solution in a water-soluble glycol. Heretofore a prepared latex was judged to be stable if it did not gel after thirty minutes in the Hamilton Beach shear test. It now appears this test is not critical enough to determine particle agglomeration and gelling tendency. A definitive shear test (HB/DCP) method to analyze latex stability was devised whereby the latex was evaluated using a disc centrifuge photosedimentometer (cf U.S. Pat. No. 4,311,039) following Hamilton Beach shear stress mixing for thirty minutes. This test more critically assesses the degree of latex particle agglomeration. It was found that often cationic latexes readily agglomerate when subjected to shear. Attempts to improve mechanical stability by increasing the cationic precursor amine monomer content or by the addition of conventional surfactants or additives, were insufficient to upgrade the latex to prevent severe agglomeration as determined by the new HB/DCP shear test method. Higher amine monomer level in the latex synthesis quite often diminishes latex conversion and increases dirt (coagulum) levels. In the instant invention, improved latex stabilization over that obtained by latex modification and conventional additives, has now been achieved by the addition of minor amounts of specific aminated epoxy resins to the cationic latex synthesized by emulsion polymerization.